Saturday, November 5, 2011

The above two images are the forecast from the new 0z NAM on the prospect of 'categorical snow'. That is, snow is possible for the area outlined. It appears that Kansas, Nebraska, Iowa, Minnesota, Wisconsin are in the possible line for snow. However, the period of snow that the NAM was forecasted for the 84th hour about a week ago did not verify, so we are investigating the chance that the end of the NAM may have a cold bias, which means that the NAM may be tilted towards colder temperatures than what will actually happen.

A strong storm system is moving north into the Dakotas region. We can see the main part of the storm is focused in Nebraska, while fronts and analysis on the bottom shows the energy combined into two lows from Colorado to South Dakota. At this time, the precipitation is in rain form for western Nebraska, which can be seen as a gray swath just to the west of the main circulation of the low.
As we progress into the night and temperatures fall, it is possible that this precipitation will turn to snow. As of right now, a sharp temperature gradient is preventing this snow from falling and is instead falling as rain, with some slight mixing out on the west Nebraska border closer to the temperature gradient.

In the last couple days, the NOAA released their winter forecast which recognized the La Nina but also called out an index named the AO that may be the 'wild card' for temperature averages this winter. I've heard quite a few people ask what the AO is.

The AO is the 'Arctic Oscillation' index. It is a varying phenomenon that exists year-round but varies the most in the cold season. Here's an image to offer a better explanation:

Left side: Positive index.

Right side: Negative index.

Image courtesy of J. Wallace

If the Arctic Oscillation is positive, the US basically sees more moderate temperatures. However, when in the negative phase, the AO often brings down colder temperatures to the US. For more advanced weather folk, below is a 500 mb heights image showing a typical AO+, AON (AO Neutral) and AO- conditions during the DJF (December, January, February) time frame.

Colors are pressure anomalies. Blue= lower pressure= colder

Orange/red=higher pressure=warmer, sunnier

Notice the swath of blue over the eastern US when the AO index is negative (bottom image), and the opposite when positive (top image).

That said, it is anticipated that we will be seeing an AO that should end up negative over the course of this winter, according to our analogues being used for this winter.

However, the AO greatly varies over a season, and this might be completely wrong. It varies just too much to be completely sure.

Another factor is the North Atlantic Oscillation (NAO). The NAO is based on pressure differences between the Icelandic Low (a permanent low pressure system stationed over Iceland) and the Azores High (a permanent high pressure stationed over the Azores). When the pressure differences are large, the NAO is considered positive. When the pressure differences are lower, the NAO is considered negative. Below is an image describing it.

The NAO has major variability and is something special to watch in the winter. Below is an image displaying the effects the NAO has on the US in the winter months.

When the NAO is positive, the Eastern US is warmer. However, when the NAO is negative, the East US experiences colder and snowier conditions as the jet stream dips southward, releasing colder air into the region. Below is a general overview of the different NAO conditions and subsequent effects thanks to Ian Bell's website:

Positive NAO Index

Negative NAO Index

More and stronger winter storms crossing the Atlantic Ocean on a more northerly track

Fewer and weaker winter storms crossing on a more west-east pathway

Warm and wet winters in Europe

Moist air into the Mediterranean and cold air to northern Europe

Cold and dry winters in northern Canada and Greenland

Milder winter temperatures in Greenland

US east coast experiences mild and wet winter conditions

US east coast experiences more cold air outbreaks and hence snowy weather conditions

Did it seem cold last year? If you thought it did, it was. Last year had a negative NAO for much of the winter months. This year, we are getting some indication that the NAO may also be negative this winter, but the specifics remain to be seen.

The South Central Plains will be making headlines this winter- and it’s not a good thing. The drought that has dried up lakes, killed crops, damaged the economy and brought Texas to its knees will only continue in a merciless act by Mother Nature. As we enter another La Nina, conditions will be favorable for a dry winter of 2011-2012. It can be expected that, with this continuing drought, wildfires will be in the headlines as well, right beside the drought. The heat will continue, and there’s no telling if the deadly high pressure ridge will return again this summer over Texas. We all hope it will not, but it’s just too far off for us to tell if it will. There is a bit of good news, though. It looks like the cooler weather and precipitation will extend a bit farther south than last winter, which theoretically increases the chances of both for the rain-starved region. But that’s theoretically. We cannot label the specifics and give a conclusion on that just yet.

The Northwest will have a slightly more exciting winter than last winter. Some details that this image does not show include that the extreme northwest (by the coast in Washington/Oregon) will likely experience cooler than normal temperatures in at least a few time frames this winter. That said, snow is likely for these regions as systems will come ashore to eventually affect the rest of the US. Montana eastward will be the focus for the main US cold region this winter. The West Coast area will be wet as well as systems continuously come onshore to eventually affect the rest of the US.

The Midwest is in for it this year. Temperatures will be below average for pretty much the entire region. More frigid air will be stationed in the Upper Midwest region extending westward into the North Plains. This frigid air will be colder than air in, say, the lower Great Lakes or Ohio Valley. Colder air, in a nutshell, makes snow ratios rise. Snow ratios are defined as one inch of water equalling ‘x’ inches of snow. Let’s say a snowstorm hit and 10 inches of snow fell in an area. Theoretically, if you were to take a column of that 10 inches and melt it into water, you would have 1 inch of water. That would be written as 1:10, or 1 inch of water: 10 inches of snow. In cold environments, snow ratios can rise to 1:20, whereas warmer climates usually have snow ratios around 1:7, defined as wet snow. With this frigid air in place over the North Plains this winter, clippers that usually dump 1-3 inches may put down over 4 inches as the colder air increases snow ratios. These clippers will also come downwards into the Midwest more often this winter. With the Canadian frigid air mass in place, some clippers may be re-directed into the Midwest. Adding to the snowfall will be ‘Panhandle Hooker’ storms- the type of storm that produced the epic February Blizzard last year in Chicago. These storms dip down south near Oklahoma and can fire up north and spread snow across the Midwest. These are usually the big snow makers for the region. The pink area we outlined as a combination of the effect cold air may have on the clippers, as well as the Panhandle Hooker storms.

The Northeast will have a widely varied winter as always with the coastal storms. However, at this point, we are thinking the majority of the region should be cooler than normal as much of the Northern Tier of the US should be as well. Areas south of the dashed lines may have a more moderated temperature average due to the wildness coastal storms can bring. This effect will be felt the most closer to the coast. Snowfall and precipitation wise, areas north of the dashed line should see snow as the big precipitation form this winter, especially for lake effect snow-prone regions. Areas in blue but south of the dashed line ought to have more variability for precipitation, but should end up on the snowier side with some ice possibly mixed in. This ice threat is more enhanced by the coast, once again due to the coastal storms. This area outlined in pink could see anything from rain to snow, and everything in between. The big news n the 2nd half of the winter will be the warm-up expected for the region. Around January, the Northeast may try and stage a dramatic warm up, greatly diminishing the chances for snow for the coastal regions. That is another reason why we expect the Northeast to get their winter early, as they did late last month (october) when we saw the early-season storm system bring over 2 feet to places in the Northeast.

The Southeast will have a similar set-up as last year’s winter. The western portions of the region will be warm and dry, with some drier weather extending along a portion of the southern area of the Southeast. In the actual Southeast, there appears to be more potential for some cool spells than last year as more frigid air will be available in the northern US than last year. The tropical threat is diminishing and will continue to do so with the tropical season coming to a close. This drier weather will be enhanced in southern Florida west ward into the panhandle of Florida, southern Mississippi, Alabama and Louisiana. The reason the states as a whole are outlined as drier is to emphasize the potential that the drier wether may extend more northerly than is projected right now. The region will be affected by the system that will eventually turn into a coastal storm later on, but these lows should remain weak while in the Gulf of Mexico. Weak, compared to what they will eventually become.

The Southwest will have a pretty interesting winter. We project the Southwest region to get a variety of temperatures and precipitation anomalies. It appears the West Coast will be wet once again as storms make their way onshore like last year, in part due to La Nina. Despite the wet Coast, the eastern areas of the Southwest will have to deal with a continuing drought and warmer temperatures also due to La Nina. This time, the warmer temperatures will extend farther west than typically experienced. As we stated in the nationwide final 2011-2012 winter forecast (click here for the nationwide forecast), we had been anticipating this for several months. We were also anticipating much below average temperatures for the Northern Plains, which brings into my head a concern: with the occasional cold hits the Southern Plains take, this frigid air from Minnesota and the Dakotas may create some damage to crops should the floodgates open from the north and pour cold temperatures down to the south.

Hello everyone, this is the much-anticipated Final 2011-2012 Winter Forecast for the US. Before we even start, let me tell you that this will be a long outlook- there's just no way around it. We have to bring in final snowfall maps, storm tracks, precipitation, temperature, ice maps over this forecast, in addition to our reasoning behind it. So if you could care less about the reasoning and just want to see the forecast, go ahead and scroll down a ways.

Let's start off with the big point that will be influencing this winter: the La Nina.
First, what is a La Nina? A La Nina is when a specific section of the Pacific Ocean has sea surface temperatures (SST's) at or below -0.5 degrees below normal. That is the definition of the La Nina. However, there are also certain patterns that can be recognized and can strengthen evidence that a La Nina is present but not immediately visible. We will not delve into those and will just go with the SST's.
For those with skills in latitude and longitude, below are the regions monitored for a La Nina/El Nino.

Let's check out what's happened in the last several months in terms of monitoring and see if you can figure out where the La Nina's are. Image runs from Jan. 1st of this year to Oct. 23. Use the coordinates in the image above to find the temperatures below for a certain region.

Blue line is sea surface temperature. Pay no mind to the light blue line.

We can see last year's La Nina in effect in January, tapering off to warmer waters during the summer, and sinking back into a La Nina in the last couple months. It is crucial to analyze past conditions in order to make a proper forecast. However, it is also crucial to analyze current conditions.That said, let's take a look at current SST's and temperature anomalies for those areas at the time of writing (October 23).

Looking at this image, we can see clearly that the coldest sea surface temperatures are in the eastern portions of this monitoring area. However, when looking at the temperature anomalies (bottom image), we see a more basin-wide La Nina ongoing. We always trust anomalies when looking for a Nino or Nina, because SST's can look cold, but not be as cold as it seems. We can see anomalies well below -0.5 degrees in this monitoring region, clearly indicating a La Nina is in place.The big question that comes with a La Nina is if it is 'east based' or 'west based'. A 'west based' La Nina is when the coldest anomalies are in the western portions of the monitoring region (top image), while an east based Nina involves the coldest anomalies at the eastern end of the monitoring area. Below are the temperature effects from West and East Based La Ninas.

The East-Based Nina favors a cooler nation, while West Based La Ninas typically torch the East Coast westwards through into the Plains. Right now, I am expecting this East Based La Nina either to stay where it is, or more likely, spread slightly westward into the center of the ENSO area.

In the last couple days, the NOAA released their winter forecast which recognized the La Nina but also called out an index named the AO that may be the 'wild card' for temperature averages this winter. I've heard quite a few people ask what the AO is.
The AO is the 'Arctic Oscillation' index. It is a varying phenomenon that exists year-round but varies the most in the cold season. Here's an image to offer a better explanation:

If the Arctic Oscillation is positive, the US basically sees more moderate temperatures. However, when in the negative phase, the AO often brings down colder temperatures to the US. For more advanced weather folk, below is a 500 mb heights image showing a typical AO+, AON (AO Neutral) and AO- conditions during the DJF (December, January, February) time frame.

Notice the swath of blue over the eastern US when the AO index is negative (bottom image), and the opposite when positive (top image).

Another factor is the North Atlantic Oscillation (NAO). The NAO is based on pressure differences between the Icelandic Low (a permanent low pressure system stationed over Iceland) and the Azores High (a permanent high pressure stationed over the Azores). When the pressure differences are large, the NAO is considered positive. When the pressure differences are lower, the NAO is considered negative. Below is an image describing it.

The NAO has major variability and is something special to watch in the winter. Below is an image displaying the effects the NAO has on the US in the winter months.

When the NAO is positive, the Eastern US is warmer. However, when the NAO is negative, the East US experiences colder and snowier conditions as the jet stream dips southward, releasing colder air into the region. Below is a general overview of the different NAO conditions and subsequent effects thanks to Ian Bell's website:

Positive NAO Index

Negative NAO Index

More and stronger winter storms crossing the Atlantic Ocean on a more northerly track

Fewer and weaker winter storms crossing on a more west-east pathway

Warm and wet winters in Europe

Moist air into the Mediterranean and cold air to northern Europe

Cold and dry winters in northern Canada and Greenland

Milder winter temperatures in Greenland

US east coast experiences mild and wet winter conditions

US east coast experiences more cold air outbreaks and hence snowy weather conditions

Did it seem cold last year? If you thought it did, it was. Last year had a negative NAO for much of the winter months. This year, we are getting some indication that the NAO may also be negative this winter, but the specifics remain to be seen.

If you have skipped down, STOP HERE.

Now we have arrived at our winter forecast. All of the above went into our forecast, and this is the best our forecast will get.

The nation will have a pretty wild winter this year. The North US will experience much above average snowfall in the pink region, which is much of the Upper Midwest, some of the Midwest, and the Great Lakes regions across the board. The Pink area will also define frigid temperatures across the winter as the floodgates holding back the truly brutal Canadian air will spill over into the far Northern Plains/Upper Midwest. The likelihood of above average snowfall for the pink area is so due to this frigid air, which may direct moisture-starved Alberta Clippers southward. In the midst of the frigid air, snow ratios (an inch of water equals 'x' inches of snow) will rise as the colder air is in place. The snow ratios fall when warmer temperatures are in place. The blue area will likely generally be cooler than average, with the potential for above average snowfall, though less than the pink area. This blue area will extend into the Northeast. Despite the early season coastal storm the region experienced, we are unsure if this will be a pattern. This is why we are monitoring the LRC. The LRC is Lezak's Recurring Cycle. The LRC states that between October 1st and November 10th each year, storms that occur in this time frame have potential to re-occur in the next 40-50 days after the original storm happens. This coastal storm was in the time frame. That does indicate that the storm may re-occur, but that does remain to be seen. There is potential for a more icy winter right by the coast during this winter. This icy area extends through the light blue area, which is an area we believe may experience cool spells during the winter. Florida may get in on this, but if they do, it will be infrequent. Florida itself out westward will be very dry, especially into drought-stricken Texas. Texas westward will also be warm, and into the West Coast will be wet as systems move onshore in an attempt to strike the East Coast/Plains/Midwest later on.

Precipitation for the winter will be much under average counts yet again for the Southern US. We anticipate this drought to be even more prone to occurring, because dry air naturally sucks out moisture from any systems. In turn, droughts create their own high pressure systems that drive away systems that may produce rain in the area. The same precipitation anomalies cannot be forecasted for the Ohio Valley, where above average precipitation will once again be dominant, similar to last year's winter. We are also extending this above average precipitation westward into the Midwest, but with a chance portion added onto the gist.

Temperatures will nosedive in the Northern Plains and Upper Midwest as the Canadian Air is released into the region, almost as if too much cold air is built up in Canada and is overflowing the floodgates, so to say. This cooler weather will extend southeast throughout the Midwest and northwards back into the Ohio Valley. However, this cooler air will not be nearly as severe compared to the North Plains brutal cold temperatures. Warmer temperatures will be prevalent across the drought-stricken region from Texas westward and northward, forming more of a boundary between Arizona and California this winter. It is possible the warmer temperatures may extend westward. The same can be said for the light blue (cooler) temperatures.

Notes: Colors have nothing to do with strength, importance, etc. and are only meant to distinguish lines apart.
The storms will be very active this winter. We will be seeing classic Panhandle Hookers come from Oklahoma through the Midwest- a few of whom may even go into the Ohio Valley. A more intense track will take a low and dip it into the Gulf of Mexico, much like dipping a strawberry into chocolate, then shooting the system north. It is important to know that the 'chocolate' (moisture) of the Gulf of Mexico will remain on the 'strawberry' (storm system) as it moves north, therefore increasing precipitation totals. It is also important to realize that the Gulf of Mexico has had a very inactive hurricane season this year. Thus, warmer waters are still on the surface- waters that would usually be mixed in with cooler, deeper waters in the Gulf if a tropical system were to move across the region. Since that has not happened, it is very possible that more moisture than normal could be pulled up into these storms that dip into the Gulf of Mexico. This track will also occur with coastal storms, which are defined by the yellow line. These storms will tread across the Gulf of Mexico, then bolt northward and hug the coast as it 'bombs out' (rapidly strengthens in a short time period). Alberta Clippers (in green) will affect the usual areas in the North Plains but will sink southward in at least several points in time. When this happens, snow is likely to occur in the more southern Midwest regions.

This year’s snowfall will be different from last year, although the pattern may be somewhat similar. We will see above average snowfall through the North Plains into the southern Midwest as some far reaching clippers and/or stronger systems that draw colder air farther south may move through the region. This above average snowfall then extends eastward as we factor in the Nor’easters the northeast region will experience this year. This coastal storm that occurred several days ago we believe has a CHANCE of being in a pattern called the LRC. The LRC (Lezak Recurring Cycle) is a pattern that sets up with low pressure systems from the beginning days of October to November. In this time frame, any of the storms that pass through could happen again 40-50 days away from when that storm happens. That is the LRC in a nutshell. More northerly, we see the much above average snowfall area. We denoted this for several reasons. The first- and probably most important- is considering that temperatures will be downright frigid in the Northern Plains. These cold temperatures will raise snow ratios. Snow ratios is the number of inches of snow you would get out compared to 1 inch of water. An average snow ratio is 1:10, with 1 inch of water equal to 10 inches of snow. In colder environments, ratios will rise, with extreme cases above 1:20. In warmer climates, like the coastal storm we saw just before Halloween this year, snow ratios will fall, resulting in heavier snow. We will likely see these Alberta Clippers sink more southerly than usual, as the frigid air may divert the storms south. Thus, the Upper Midwest south would get the heavier snows. Another reason is for the Panhandle Hooker storms. Panhandle Hookers are storms that hook around the Oklahoma Panhandle and shoot north into the Midwest, usually bringing ample moisture, leaving behind fair amounts of snow. This also affects the Ohio Valley, which is why we have included them in this above average snowfall. Colorado Lows are similar, but curve around Colorado and go north. These typically affect the Plains more than anybody for snowfall.

The national ice threat will once again be similar to last year as temperature boundaries initiate a battle zone when systems move through the region. This ice threat will also occur with Nor'easters that may bring up some warmer air from the South.

Thank you very much for reading this. A lot of time and effort went into this, and we hope you liked it. The releases are not done yet...

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